The present invention relates to a high-strength spring steel with its crystal grains becoming finer from the core to the surface layer and the crystal grains in the surface layer being extraordinarily fine, which is obtained by repeatedly heating for short time intervals the surface of a steel material so as to heat the steel material throughout the entire steel body or bring about a heating condition close to it, by a high-frequency induction heating method or the like, followed by quenching and then tempering by high-frequency induction heating or the like.
To be more specific, the high-strength spring steel according to the present invention is obtained by heating the surface of the steel material to over the AC.sub.3 transformation point and then decreasing the surface temperature of said steel to below the Ar.sub.1 transformation point after stopping the heating, this cycle of rapid heating and cooling of the steel being successively repeated until a thorough heating of the entire volume of the steel piece or a condition close to is is brought about, which is followed by quenching of the steel.